1. Field of the Invention
The present invention relates to a correlation detection device, its method and a communication terminal device for detecting special codes contained in an input signal and is suitably applied to the case of setting the timing using special codes especially in the wireless LAN to connect in a wireless fashion the plural number of terminal devices.
2. Description of the Related Art
In recent years, with the development of high technology in computers, LAN (local area network) has been formed by connecting multiple computers and files and data are used in common and electronic mails and data are transmitted extensively. In the conventional LAN, computers are connected via cables such as using optical fiber, coaxial cable, or twisted pair cable each other.
However, in the wired LAN, it is very difficult to install the LAN since the installation work is necessary in order to connect these an such wired LAN requires very complicated cabling. Thus, the wireless LAN has become a focus of attention to solve the wiring problem of the conventional LAN cable system.
As the wireless LAN, the system that performs data communications according to CDMA (code division multiple access) system using spread spectrum has been proposed. In this CDMA system, the transmission data is multiplied by the PN code (pseudo noise code) and spectrum of the transmission data will be spread. The data transmitted upon being spectrum spread is demodulated multiplying by the same PN code as the transmitting end. This CDMA system has an excellent interference resistivity as well as having adequate security.
In recent years, with the dissemination of information through multimedia, data large in size such as video and audio have been widely handled. Accordingly, it is required to increase the transmission rate in order that a large volume of data such as video and audio can be transmitted in the wireless LAN. However, in the spread spectrum modulation, if the data is transmitted at a high velocity rate such as approximately 30 [Mbps], the bandwidth more than 300 [MHz] becomes necessary. Such broad band cannot be secured according to the present frequency allocation, and also it is very difficult to communicate using such broad band.
Moreover, in the spread spectrum synchronization recovery time for matching the phase of data code transmitted and the phase of code to be generated at the receiver for demodulation is necessary when demodulating. Accordingly, in the spread spectrum, a sequence of bits for synchronization is inserted into each packet in order to get synchronization at high speed, and due to such bits sequence for synchronization, it causes a problem that bits other than effective data would be increased.
Hence, the application of this patent proposes that data will be transmitted by the OFDM (orthogonal frequency division multiplexing) scheme, as well as the data communications will be conducted by the TDMA (time division multiple access) scheme making one frame as a unit, and M-sequence code will be transmitted at the head of one frame, the transmit/receive timing will be set relative to this M-sequence code, and the transmit/receive timing of each communication terminal device will be specified by the control information from the communication control terminal device. Since in this OFDM scheme, data will be transmitted in parallel utilizing the plural number of subcarriers orthogonally arranged, transmission rate can be increased easily and if jitter occurs, it can be demodulated without error. Also, at the time when receiving, since transmit/receive timing is set relative to the M-sequence code on the head of one frame, data can be reproduced by demodulating only the necessary symbol in the frame using this time information.
Accordingly, in the case of receiving the M-sequence code and setting the timing, it is necessary to detect the M-sequence code from the receiving signal. As the circuit for detecting such M-sequence code, utilization of a correlation detection circuit using matched filter can be considered.
At this point, the correlation detection circuit having a matched filter will be described referring to FIG. 1. As shown in FIG. 1, a receive signal S1 is entered into a matched filter 2 via an input terminal IN in the correlation detection circuit 1. The matched filter 2 is made up of digital filter such as FIR filter, and as shown in FIG. 2, it is comprised of delay circuits D.sub.1 -D.sub.n, multipliers K.sub.1 -K.sub.n and an adder A.sub.1. The coefficient, that multipliers K.sub.1 -K.sub.n multiply, will be set to "1" or "-1" according to the code to be detected. And if the correlation between the code set to the coefficient of multipliers K.sub.1 -K.sub.n and the input code received is strong, the output level of the adder A.sub.1 becomes large.
Then, the matched filter 2 transmits an output signal of this adder A.sub.1 to a comparator 3 of the succeeding stage as a correlation value signal S2. The comparator 3 compares the signal level of the correlation value signal S2 to be transmitted from the matched filter 2 and the signal level of the threshold signal TH to be supplied from the outside, and if the signal level of the correlation value signal S2 is larger than the threshold signal TH, the comparator 3 outputs a detection output S3 showing that the signal having correlation has been received via an output terminal OUT.
In the case where the M-sequence code is supplied as the receive signal S1, output of the matched filter becomes large and since the signal level of the correlation value signal S2 becomes larger than the threshold signal TH, the detection output S3 showing that the M-sequence code has been received is sent out from the correlation detection circuit 1.
However, in the correlation detection circuit 1 constructed as described above, there are cases where detection output S3 would not be sent out even when the M-sequence code is received but if the signal level of the signal other than the M-sequence code received immediately before is large.
In general, at the preceding stage of the correlation detection circuit 1, AGC (automatic gain control) circuit is provided and this adjusts the receive signal S1 to the prescribed power. As described above, there are cases where the detection output S3 would not be output because of this AGC circuit even when the M-sequence code is received. For example, as shown in FIG. 3A, the M-sequence code is simply imbedded in the other signal even if the AGC circuit functions, uniform power adjustments will be conducted. And thus the correlation value signal S2, that is the output of the matched filter 2, increases when the M-sequence code is received. And as a result, the detection output S3 showing that the M-sequence code is received normally can be sent out by the correlation value signal S2 exceeding the threshold TH.
However, as shown in FIG. 3B, when the other receive signal received immediately before the M-sequence code is large, the processing to decrease the gain is conducted by the AGC circuit. However, since the recovery time is required for the AGC circuit to increase the gain, there are cases where the low gain state continues for some time even when the M-sequence code is received. Accordingly, the receive signal of the M-sequence code will not be amplified and as a result, the output of the matched filter, the correlation value signal S2, will not exceed the threshold TH, and there are cases where the detection signal S3 would not be transmitted even though the M-sequence code has been received.
Furthermore, if the receive signal other than the M-sequence code received immediately before is extremely large, there is a possibility that the correlation value signal S2, which is the output of the matched filter 2, outputs the detection output S3 exceeding the threshold TH by mistake even though the M-sequence code has not been received.